Ginkgoes are often planted in cities not only because they're pretty trees but also because they thrive where air pollution is bad. It's not surprising that ginkgoes are air-pollution tolerant, because they are very primitive plants; they may have evolved when the earth's atmosphere was even more sulphurous and grimy than today, because of erupting volcanoes. In fact, ginkgoes are living fossils.
During early stages of the evolution of trees and tree-like plants, ginkgoes were much more common than now. The oldest ginkgo fossils appear in rocks of the Permian Period, deposited some 280 million years ago, long before the first dinosaurs and flowering plants had appeared. By Jurassic times, the dinosaurs' heyday, there were many ginkgo species and even entire genera in the ginkgo family and order that now are extinct. By the Cretaceous Period, some 135 million years ago, about when the first flowering plants appeared, but before the dinosaurs disappeared, already many ginkgoes and ginkgo-like species were going extinct. By the Tertiary Period, from about 63 to 13 million years ago, of the entire Ginkgo order with its many genera, there remained only a few species of the genus Ginkgo. Today, there's just one ginkgo species left, the one that grew in Brooklyn, Ginkgo biloba.
In fact, it's not at all certain that even today ginkgoes could survive without man's help. For a long time Ginkgo biloba was considered as extinct as all the others, but a century or so ago it was discovered growing in Asia. Seed were collected, distributed, and today ginkgoes are common street trees. It's unclear whether they survive in the wild in Asia.
If ginkgoes are so primitive, they must have retained some features we don't see much nowadays. What are they?
For one thing, unlike most gymnosperms, ginkgoes drop their flat-bladed leaves in the fall -- they're deciduous. At the left you see a close-up of a ginkgo leaf collected in the fall, showing its characteristic yellow fall-color. The ginkgo's leaves are unlike any other deciduous leaf. Elsewhere we see that the leaves of flowering plants are usually either net-veined, or parallel veined. The veins of ginkgo leaves are neither. As you can see if you look closely at the image at the left, ginkgo-leaf veins arise from the bottom of the leaf blade, then fan out toward the rim of the butterfly-shaped leaf,dividing as they go. Therefore, the veins aren't really parallel to one another, and since the vein tips don't connect with other veins, they're not really net-veined.
Ginkgo sex organs are just as strange as ginkgo leaves. Pollen is produced in curious, dangling structures, and the female parts are nothing like cones. Each female structure consists of ovules borne in pairs at the tips of delicate stalks. In other words, they're about as "naked" as they can be, which is what would be expected of a very primitive gymnosperm.
The puckered ginkgo fruits shown in the image at the top of this page can be a little confusing, for the fruits look as if they are flesh just like a plum or peach, which are angiosperms -- and we've seen that in all angiosperms the seeds are imbedded in the old flower's matured ovary wall. Well, what's happened with the ginkgo is that once its naked ovules are pollinated, the ginkgo's seed-coats turn fleshy. So, it's the seed-coats providing the ginkgo fruit's fleshy part, not the ovary wall, as in angiosperms.
The flesh surrounding ginkgo seeds stinks like crazy, by the way. Also, notice that the ginkgo has separate male and female flowers. It seems that in almost every respect ginkgoes are abnormal and exotic. If you can find a ginkgo, just sit down and admire it. When you examine it, keep in mind that you're really looking at a tree type that was already "outdated" when the first dinosaurs appeared.
If now you are bitten by the ginkgo bug, you can't do better than to visit The Ginkgo Pages on the Web.
Actually, Ginkgo trees have become rather famous lately because of their reputed medicinal value. Probably there isn't a "health store" in the country without a few packages of "Ginkgo biloba" on their shelves. If you want to check out the "ginkgo biloba phenomenon," look at Amazon.com's Natural Care Library Gingko: Safe and Effective Self-Care for Headaches, Depression and Circulation.